1. Field of the Invention
The present invention relates to a multi charged particle beam writing apparatus and a multi charged particle beam writing method. For example, the present invention relates to a method of obtaining high accuracy of multi-beam writing.
2. Description of Related Art
The lithography technique that advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. The electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for writing or “drawing” a pattern on a wafer, etc. with an electron beam.
As an example using the electron beam writing technique, there is a writing apparatus using multiple beams (multi-beams). Compared with the case of writing using a single electron beam, since a multi-beam writing apparatus can emit multiple radiation beams at a time, it is possible to greatly increase the throughput. In such a writing apparatus of a multi-beam system, for example, multiple beams are formed by letting an electron beam emitted from an electron gun assembly pass through a mask with a plurality of holes, blanking control is performed for each of the beams, and each unblocked beam is reduced by an optical system and deflected by a deflector so as to irradiate a desired position on a target object or “sample” (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2006-261342).
In order to maintain writing accuracy, it is required to calibrate the amount of beam current. In the single beam system, especially in the variable shaping system, since the shot size varies by shot, what is necessary is just to adjust the current density of a beam to be uniform. On the other hand, in the multi-beam system, unlike the single beam system, since the variable shaping is not performed and the shot size of each beam is fixed to be the same size, it is necessary to perform adjustment so that each current amount may be constant. In order to maintain writing accuracy, the beam current amount needs to be calibrated. In the single beam system, since the beam area to be used as a shot in a beam emitted from the electron gun assembly is small, the current density in this area can be almost uniform. However, since many beams in a large area are used in the multi-beam system, it is difficult to make each current uniform. Therefore, it is necessary to correct the irradiation time according to variation of the current amount of each of multiple beams.
In the single beam system, the amount of beam current can be measured by applying irradiation of a beam to the Faraday cup on the stage. Since only one beam is used in the single beam system, measuring can be performed before and after writing processing. However, unlike the single beam system, since a large number of beams are used in the multi-beam system, it may take several days to measure all of the current value of each beam by using the Faraday cup on the stage. Therefore, the writing processing is stopped during the measurement, thereby causing problems of reduction of the apparatus operation rate and reduction of the writing throughput.
Moreover, current density distribution of multiple beams may always change due to temporal change of the cathode of a beam emission source, such as an electron gun assembly, therefore, there is a problem that the current amount of each of the multiple beams will also change.